Photovoltaic solar electricity is produced by means of a converter of the light of the sun into electrical energy. Today, different types of solar cells achieve this conversion at different levels of efficiency. For example, commercial crystalline flat-panel silicon solar cells attain 15% to 20% efficiency, depending upon the manufacturer, without concentration. More sophisticated (and expensive) multi-layer cells are more efficient, particularly working under concentrated sunlight. In the case of silicon solar cells, the most efficient are the Back-Point-Contact (BPC) cells, which have gone above 27% cell efficiency, under irradiance levels around 10 W/cm2.
The highest efficiencies have been obtained with solar cells based on III-V semiconductor materials, with commercial cells in the 35-40% range, formed by three current-generating junctions stacked in tandem in a monolithic configuration, grown by Metal-Organic Chemical Vapor Deposition (MOCVD) on a semiconductor substrate. Today's standard commercial process uses a Germanium (Ge) substrate on which are grown a stack of lattice-matched semiconductor layers: a Ge bottom junction, a Ga(In)As middle junction, and a GaInP top junction. The higher the desired efficiency, however, the greater the number of layers and hence the higher the cost. Today, bare cell costs are in the $100-200/m2 range for thin film solar cells, $300-500/m2 for flat-panel silicon cells, $20,000-40,000/m2 for BPC cells, and $50,000-100,000/m2 for III-V multi-layer solar cells. Therefore, the electricity produced by the very high efficiency cells (such as the triple junction cells) cannot be competitive at one sun irradiance with electricity produced by cheaper low efficiency cells (except in outer space). Only when an optical concentrator reduces the necessary cell area does cost competition become possible. Typical geometrical concentration ratios (i.e., ratio of the entry aperture area to the cell area) for high-efficiency high concentration silicon and III-V solar cells range from 200 to 1,000.
The high concentration required for these high efficiency cells produces correspondingly higher current densities as compared to one-sun cells. In the case of III-V solar cells, designing for the highest efficiency requires an optimization that balances the characteristics of the various layers so they all work well on the identical current going through them. In the case of III-V solar cells, these parameters include the series resistance joule losses, the recombination at the metal-semiconductor ohmic contacts, and the front metal grid shading factor. In today's optimized III-V solar cells, the main cause of losses in the device is grid line shading, which is about 8-10%.
As an example of suitable dimensions, the gridlines may be 5 μm high, 10 μm wide at the base, and spaced apart at 75 μm center-to-center pitch. The gridlines may be formed by photoresist etching.
Recovering the light reflected by the grid lines has been a subject of prior art. U.S. Pat. No. 4,711,972 by O'Neill disclosed refractive microlenses aligned over the grid lines to refract the light away from them. Such small devices, however, are difficult and expensive to align over the gridlines. These microlenses, further, are not effective under wide-angle illumination (as occurs in high concentration systems), especially when a secondary optical element (SOE) is optically coupled to the cell as a concentrator or homogenizer.
Another approach, in principle compatible with wide-angle illumination, is optically coupled secondary optical elements (SOE), which was suggested in Chapter 14 of Luque's 1989 book “Solar Cells and Optics for Photovoltaic concentration.” In this approach, the grid lines must be shaped to reflect the light impinging on them towards the uncovered semiconductor (A. Luque et al., Progress in Photovoltaics: Research and Applications, Volume 12, Issue 7, Pages 517-528, 2004). Similarly, TIR vs.-groove microconcentrators have been proposed (EU project Euclides, also Omer Korech et al., Optics Letters, Vol. 32, Issue 19, pp. 2789-2791, 2007). In these microconcentrator concepts, the recovered light eventually reaches the semiconductor, but at very high incidence angles, for which the cell reflectivity is high (in spite of the cell surface being antireflection (AR) coated).
Finally, U.S. Pat. No. 5,291,331 by Miñano and Luque disclosed rotationally symmetric elliptical reflectors as external reflective angular-confining cavities that collect the light reflected by the grid lines (as well as Fresnel reflections from the semiconductor front surface) of one cell and then redirect it back to that cell or to another cell placed nearby.
Multiple combinations of silicon and III-V solar cells with such cavities have been considered (A. Luque, G. Sala, J. C. Miñano, P. A. Davies, I. Tobías, J. Alonso, C. Algora, G. L. Araújo, J. M. Ruiz, A. Cuevas, J. Oliván, P. Dunn, G. Rice. J. Knobloch, B. Voss and C. Flores: The photovoltaic eye: a high efficiency converter based on light trapping and spectrum splitting, 10th European PV Solar Energy Conf., Lisbon, 627-630 (1991).
Spectrum splitting is discussed by E. Shifman in U.S. Patent Applications Nos. 2008/0000516 and 2005/0046977, in an on-axis Cassegrain concentrator with a parabolic primary mirror and a hyperbolic secondary mirror. The spectrum-splitting filter is on the secondary mirror, and the two cells are located in parallel planes, facing one another on opposite sides of the secondary mirror.